To me "nightwatch" is the darker one on the watch, so the other ones like G!G! will have a lighter mood. I didn't want to do another Rembrandt tribute, as it was already well done, and it wouldn't have any interest at all to do it again.
In fact, I've chosen to represent the two Vimes (old and young), Nobby and Colon old, in the opposite situation. In my humble opinion the book is about the dark side of Vimes and how he could have evolved...
The position of the two Vimes is a small reference to Leonard the Vinci (he had a slight confusion between his "babysitter" and his mother):
http://fr.wikipedia.org/wiki/Fichier:Le ... native.jpg

Here are the main reasons, awfully explained in my horrible English :]